Photoelectric cathode



Feb. 6, 1940. E. FLORY PHOTOELECTRIC CATHODE Filed Jan. 19, 1937 I4 IIIIIIIII/l III) III/[I'll Jnnentor (lttorneg Patented h s, 1... 2,139,322

UNITED STATES PATENT OFFICE PHOTOELECTRIC CATHODE Leslie E; Flory, Oaklyn, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application January'19, 1937, Serial No. 121,259

8 Claims. (01.250-165) My invention relates to photoelectric cathodes comprising a plurality of resistors connected in and particularly to cathodes of the type which are ample, in the image amplifier described and claimed in application Serial No. 55,957, filed December 24, 1935, in the name of George A. Morton and assigned to the Radio Corporation of America, the'cathode should be transparent whereby an optical image projected on one side of the cathode will release electrons on the opposite side thereof. An object of my invention is to provide an improved cathode of the above-mentioned type.

A further object of my invention is to provide an improved method of making a cathode of the above-mentioned type.

In practicing my invention, I form a transparent layer of silver oxide on a transparent supporting surface, thensensitize the silver oxide layer by caesium or the like, and finally form a transparent layer of silver on the sensitized surface. The final layer of silver gives the cathode the necessary conductivity (the base layer of silver oxide being an insulator) and also increases the photo-electric sensitivity.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a view of an image amplifier tube embodying my invention,

' Fig. 2 is a view taken on the line 22 of Fig. 1,

and

Fig, 3 is a view in cross-section on an enlarged scale of the cathode shown in Fig. 1.

In, Fig. 1, I have shown my. invention applied to an image amplifier tube of the type described and claimed in the above-mentioned Morton application. This tube comprises a highly evacuated glass envelope I having a semi-transparent fluorescent screen 2 on one end thereof. A transparent photoelectric cathode 3 hereinafter described is located at the other end of the glass envelope.

Located coaxially along the tube between the cathode 3 and the screen 2 are a plurality of focusing rings 4, 6, I, 8 and 9, and an anode II which may be in the form of a foraminous cylinder. A suitable voltage source such as a battery l2 maintains the anode l I highly positive with series. One end of potentiometer I3 is connected to the cathode 9 through a metallized edge 5 and to the negative end of battery l2, while the other end of the potentiometer is connected through a 5 variable tap to a'point on a potentiometer l4 shunted across battery 12.

As described in the above-identified Morton application, flanges or discs l6 and I! are supported from and electrically connected to the 10 ring 9 and the anode I I, respectively, for the purpose of producing the'desired magnification of an image projected upon cathode 3 and appearing upon screen 2.

In accordance with one feature of my invention,

I attach an inner shielding ring i8 to the disc I6 and support a circular filament I9 between the shielding ring l8 and the focusing ring 9 as shown more clearly in Fig. 2. Silver beads 2| are located at intervals on the filament I9 whereby silver may be vaporized by heating the filament. The shielding ring 18 tends to keep the silver off the rings 4, 6, l and 8 and the resistors l3 and to direct the silver vapor toward the cathode 3 where it forms a silver layer thereon during the process of making the cathode.

The specific structure of the cathode 3 and the method of making it will now be described. Referring to Fig. 3, the cathode comprises a transparent supporting member 26 which preferably is 30 of glass or mica. In the particular tube illustrated it is a circular convex member similar to a watch crystal. The photoelectric surface on the member 26 consists of three more or less distinct layers, the first one being a transparent layer 21 of the several layers on the glass surface may not 0 be strictly correct as it is difiicult to determine the exact nature of such structure, but it is believed to be substantially correct.

The nature of the cathode 2 will be better understood by a description of the way in which it is produced. The photoelectric surface is formed on the supporting member 26 after the tube shown in Fig.1 has been completely assembled. First, the assembled tube is baked and pumped to a high vacuum. Next, a thin film of silver is formed on the surface of member 26 by heating filament l9 to evaporate silver from the silver beads 21. This silver film is about 25% transparent.

The next step is to introduce oxygen into the 5 envelope l and oxidize the thin silver layer. The oxldization may be produced by the use of a high frequency discharge in the tube as described in Essig Patent 2,020,305. The resulting silver oxide layer 21 is about to transparent and is an .insulator.

The envelope I is again pumped to a high vacuum and the silver oxide layer 21 is next sensitized in the usual manner. This consists in admitting caesium or some other alkali metal from a side tube (not shown) and then baking the complete tube to cause the caesium to react with the silver oxide. This results in a layer 28 which probably is caesium oxide with a thin layer of free caesium on the surface.

If sufficient caesium has been added, the film may be conducting and will show fair photoelectric sensitivity. However, the stability of such a film as regards conductivity is poor.

To overcome this defect, the next step is to evaporate from the beads 2| a very thin transparent layer 29 of silver upon the sensitized surface. The final step is to again bake the tube, this baking being similar to the previous one which was used to cause a reaction between the caesium and the silver oxide. This results in a large increase in photoelectric sensitivity and, at the same time, the additional silver layer 29 adds the necessary conductivity to the photoelectric surface.

It will be understood, of course, that the il1ustration in Fig. 3 is diagrammatic in that the various layers on the member 26 are too thin to be shown in their true thickness. Also, it is likely that the several layers are intermixed to a certain extent not indicated on the drawing.

From the foregoing description it will be apparent that by oxidizing completely through the first silver layer and allowing this oxide to react with caesium, we have the active photoelectric layer directly accessible to light passing through the member 26. At the same time We have a very sensitive surface which is conducting because of the outside silver layer.

I claim as my invention:

1. An electric discharge tube comprising an envelope having therein a transparent cathode supporting surface, a focusing electrode and an anode located in the order named along the axis of the tube, said focusing electrode com prising two concentric rings having the space therebetween closed on the side facing said anode by a washer-like disc forming an annular recess facing said cathode surface, and a filament mounted in said recess and having silver thereon which is located about said annular recess whereby silver may be evaporated upon said cathode supporting surface.

2. The invention accordingto claim 1 wherein the silver on said filament is in the form of beads which are substantially uniformly spaced around said annular recess.

3. An electric discharge tube comprising an envelope having therein a transparent cathode supporting surface and a plurality of annular electrodes located along the axis of said tube, one of said electrodes comprising two concentric annular surfaces having the space therebetween closed on the side opposite said cathode supporting surface by a washer-like disc forming an annular recess facing said cathode surface, and a filament mounted in said recess and having a metal thereon which vaporizes at asubstantially lower temperature than said filament whereby said metal may be evaporated upon said cathode surface.

4. An electric discharge tube comprising a transparent photoelectric cathode adapted to have a light image formed on one side thereof whereby a corresponding image of electrons is released from the opposite side thereof, said cathode comprising a transparent supporting surface, a transparent layer of silver oxide on and in contact with said surface, a transparent photoelectric sensitized surface on said silver oxide, a transparent layer of metal on said photoelectric surface which layer provides the necessary conductivity for said cathode to avoid distortion in the focusing of said electron image, and means for focusing said image of electrons in a plane removed from said cathode.

5. The invention according to claim 4 wherein said layer of metal is a layer of silver.

6. An electric discharge tube comprising a transparent photoelectric cathode adapted to have a light image formed on one side thereof w r y a corresponding image of electrons is released from the opposite side thereof, said cathode comprising a transparent supporting surface, a transparent layer of silver oxide onsaid surface, said layer being photo-sensitive on the surface farthest removed from said supporting surface, a transparent layer of silver on said photo-sensitive surface, and means for producing an electrostatic focusing field for focusing said image of electrons in a plane removed from said cathode, said layer of silver being thick enough to give said cathode sufficiently good conductivity to prevent distortion of said focusing field near the cathode due to a potential gradient along the cathode surface.

7. The invention according to claim 6 wherein said layer of silver oxide has been photo-sensi-- tized by means of an alkali metal whereby there is a transparent alkali metal oxide surface on said silver oxide.

8. The invention according to claim 6 wherein said layer of silver oxide has been photo-sensitized by caesium whereby there is a transparent caesium oxide surface on said silver oxide.

LESLIE E. FLORY. 

